Pipelines are used to transport fluids such as the production fluids from oil and gas wells. Because the measurement of these fluids is important, orifice plates are installed in special fittings which are installed in-line with pipeline sections. Some fittings may permit an orifice plate to be moved in and out of the flow stream without interruption of the flow through the pipeline. Other fittings permit orifice plates to be moved in and out of the pipeline only by interruption of flow.
The use of orifice measurement for flow has been known since ancient times. The basis of orifice measurement is to place a plate in a flow line, with the plate having an opening which is smaller than the opening of the flow line. By reading the upstream and downstream pressure on either side of the plate, and calculating the difference of pressure between the upstream and downstream pressures, one can infer the rate of flow in the pipe line.
The accuracy of the measurement given by the orifice is dependent on many factors, including the ratio of the orifice hole to the diameter of the pipe, the length of straight run of the upstream and downstream pipe or tube sections on either side of the orifice, the eccentricity of the orifice hole in the pipe or tube, leakage of fluid around the plate, and the like.
As part of the effectiveness of an orifice plate, it is important to seal around the circumference of the plate and to effect a seal within the interior of the orifice fitting to ensure that all the flow passes through the plate bore and is measured. Accordingly, the sealing surface of the seal must seal against the machined seat in the orifice fitting, typically on the downstream side. In the prior art, the seal slot in the fittings is held to a closely-machined tolerance. However, variations to the slot may occur because of casting variations in the sand castings used as raw material for orifice fitting bodies. Also, sometimes fittings are subjected to unusual loads in pipeline applications and some seat expansion or deflection can occur. Further, old fittings in use are always subject to erosion or corrosion and therefore pitting in the sealing surfaces. In the prior art, standard, rounded (or, in some cases, flat) seal contours are used. These contours are not forgiving in nature. Displacement of a solid, molded piece of elastomer is not always easily accomplished. It is a necessity to enter the slot as originally machined without undue friction to avoid damaging the seal. In such a case corroded seats also prevent a uniform flow of rubber and result in the sealing problem as the rubber is unable to conform to the sealing surface of such a corroded seat.
Accordingly, because the object of an orifice fitting is to hold and effect the seal on an inserted orifice plate and seal assembly, the deflection referenced above, caused by internal pressure or other distortions, will allow some seal leakage in certain instances. Therefore, because the sealing faces of the seats within the fitting are machined parallel, any deflection that causes a "non-parallel" seat face deflection can contribute to leakage around the seal and seat face. Thus, the seal design of the prior art of the single rounded, or flat, elastomer surface to contact the fitting seat places a large mass in the seal "bead" and compresses only with considerable force. Increasing the width of this style seal to allow for the widening of the seat gap under pressure creates unacceptable difficulty in inserting the "thicker" seal in a non-pressurized fitting where there is no seat gap expansion.
There is, therefore, need for an orifice plate and seal combination that permits sealing even in the presence of distortions in the slot and abrasions in the sealing surface.
In other art, such as the molded seal incorporated into the bottom of the piston of a Daniel's piston-controlled check valve line, non-bead or flat seals have been used. This enhances sealing under low pressure conditions and when "sand" from flowing wells is encountered. This type of molded seal is a modified three-lobe design and has been used in the prior art for special applications. Another seal that does not use a single bead or flat design is that designed by Parker, designated a Gask-O-Seal, a molded rubber-to-metal gasket used in the body or top closure of certain high pressure (2500 ANSI) Daniel's senior fittings in the prior art. This molded seal is not a three-lobe design.
Further, there has been a series of meetings to significantly revise API/AGA standards, which would constitute a revision to Manual of Petroleum Measurement Standards, Chapter 14, "Natural Gas Fluids Measurement", which includes Section 3, "Concentric, Square-edged Orifice Meters". The revised Section 3 may become an update and would then become a revision to ANSI Standard ANSI/API 2530; i.e.: AGA Report No. 3.
These new requirements have been adopted by some companies even though additional costs will be attendant with mechanical changes associated with the requirements. As discussed above most orifice plate holding devices other than flanges require some manner of plate/seal/carrying device combination. The most common seal of this type is the elastomeric seal which is assembled around the orifice plate. Industry standards have changed in respect to this seal.
Prior to the development of the seal of the present invention, field tests using actual conditions showed that prior art seal profiles sealed bubble tight at a differential pressure as low as 25 inches of water. However, the changes to the API 14.3 standard discussed above which covers seals of this type have moved customers to request fittings that seal bubble tight at as low as 5 inches of water. The theory of operation of the existing seal profile of the prior art is that the elastomer deforms to any irregular profiles in the fitting seat face to effect a sealing engagement. The differential pressure created as the seal makes up in the initial condition causes a net force in the direction of decreasing pressure, thus forcing the seal to take the profile of the fitting seat to stop any fluid flow around the seal. The seal must thus be responsive to lower differential pressure to meet new industry demand.
Therefore, an object of the present invention is to operate as a bubble tight seal with minimum differential pressure of as low as 5 inches of water but still be able to operate at high maximum differential pressures, such as 200 inches of water.